(1) FIELD OF THE INVENTION
The present invention relates to an optical scanning device designed such that an image forming optical beam to form a latent image on a photoreceptor member and a synchronous signal generating optical beam generated independently thereof are reflected on a same reflecting face of a rotary polygon mirror.
(2) DESCRIPTION OF THE PRIOR ART
An optical scanning device employed in a laser beam printer and the like has a rotary polygon mirror for scanning optical beam. A problem with such device is that lines to be scanned by the optical beam are not always aligned with one another in a sub scanning direction because of some manufacturing errors in angle dividing precision of the reflecting face of the rotary polygon mirror or because of irregularities or vibrations associated with the rotation of the rotary polygon mirror. One prior art method attempting to overcome this problem is as follows: A light receiving section for detecting the optical beam is disposed on a scanning line more upstream than a photoreceptor member. When a predetermined time period has elapsed after the light receiving section detects the optical beam, a modulation of this optical beam based on an image signal is started, thereby to synchronize a timing of the start of the modulation. Accordingly, the latent image formed by scanning by a plurality of times the optical beam on the photoreceptor member moved in the sub scanning direction can be aligned with the sub scanning direction.
However, it is necessary to dispose the above-described light receiving section for detecting the optical beam and for controlling the synchronization of the modulation so as to be away from the latent image forming area on the photoreceptor member. In the case of a construction using the same optical beam both for forming the image and for synchronizing the start timing of the modulation, it takes considerable amount of time for the optical beam to reach the latent image forming area after being detected by the light receiving section. That is to say, additional scanning of the optical beam is needed for generating the synchronous signal, thereby to slow down the image forming operation.
One prior art method for overcoming this problem is disclosed in Japanese patent application laid open under NO. 53-3833, in which independent optical beams are used respectively for forming the image and for generating the synchronous signal thereby eliminating the additional scanning operation and consequently speeding up the image forming operation. However, according to this method, in connection with the rotation of the rotary polygon mirror, the optical beam for generating the synchronous signal scans also the latent image forming area on the photoreceptor member. This results in the photoreceptor member also being exposed to the optical beam for generating the synchronous signal, causing noises in image information leading to image deterioration. Therefore, there has been desired an optical scanning device with which the synchronous signal generating optical beam does not disadvantageously affect the image.